1. Field of the Invention
This invention relates to the field of paper manufacturing and, more particularly, to maintaining the brightness of bleached pulps or paper containing lignin. Maintaining brightness is also known in the industry as retarding or reducing brightness reversion.
2. Description of the Prior Art
Pulp is the raw material for the production of paper, paperboard, fiberboard and the like. It is obtained from plant fiber such as wood, straw, bamboo and sugarcane residues. Wood is the source of 95% of the pulp fiber produced in the United States. Dry wood consists of 40 to 50 percent cellulose, 15 to 25 percent other polysaccharides known as hemicelluloses, 20-30 percent lignin, a biopolymer which acts as a matrix for the cellulose fibers, and 5 percent of other substances such as mineral salts, sugars, fatty acids, resins and proteins. Lignin is composed primarily of methoxylated phenylpropane monomeric units interconnected by a variety of stable carbon-carbon and carbon-oxygen (ether) linkages. The color of paper produced from pulp arises from the lignin.
Paper or pulps containing lignin or lignocellulose are commonly dark and must be bleached if a white paper is desired. A major drawback of bleached lignin-containing pulps is that they are easily and extensively darkened by light irradiation. This limits their use in various grades of printing papers.
Paper or pulp used in making newsprint is generally not bleached. If it is bleached, it is relatively mildly bleached as compared to higher quality paper. Therefore, such unbleached or mildly bleached pulps have a darker quality than bleached pulps. Newsprint has a relatively high lignin content and, therefore, has a tendency to become even darker when exposed to light. The present invention can be used to prevent darkening of all types of lignin-containing pulps including bleached and unbleached pulps, such as newsprint.
The whiteness of paper is estimated by brightness measurements which are based on the reflectance of light having an average wavelength of 457 nm. An Elrepho brightness meter is one type of instrument used to measure paper brightness. A low brightness (40% Elrepho) indicates brown or dark paper, while 90% Elrepho typifies white paper. Lignin-rich pulps have brightness values in the range of 50-70% Elrepho, depending on the wood species used and the pulping process. These pulps can be bleached to 70-90% brightness using known brighteners such as hydrogen peroxide, sodium borohydride or sodium dithionite. Hydrogen peroxide is normally used when a brightness of more than 70% is required. A problem with bleached lignin-rich pulps is that they may darken by as much as 20 Elrepho points when exposed to natural sunlight during exposure over a period of only one day.
Several methods are presently known to decrease brightness reversion in pulps containing high levels of lignin. The disadvantage of these methods is that they add significantly to the cost of the paper manufacturing process and are less effective than the method of the invention.
One known method described in European Patent No. 0 280 332 (Agnemo et al.) consists of serial treatments to reduce the carbonyl groups (photosensitizers) contained in the pulp to alcohol groups. In addition, the process includes alkylation of the phenolic hydroxyl groups in the lignin, from which hydrogen atoms are abstracted, by the use of an alkaline propylene oxide. Addition of fluorescent compounds that absorb or reflect the ultraviolet light which would otherwise excite photosensitizer groups is also disclosed.
It has recently been discovered that compounds with a labile hydrogen atom can significantly decrease the rate of darkening of high-yield pulps when they are irradiated with ultraviolet light. Effective anti-reversion agents include ascorbic acid, sulfoxylates, thiols and 2,4-hexadien-1-ol. Our own earlier U.S. Pat. No. 5,080,754 describes the use of formyl compounds as additives which donate hydrogen atoms to reactive intermediates created in the bleached pulp upon exposure to light.
All known additives suffer from the drawback that while they inhibit light-induced reversion, they accelerate the yellowing that occurs on routine storage out of the light (thermal reversion). Thermal reversion is another phenomenon that limits the use of high-yield pulps in certain grades of paper.
While not wishing to be restricted to a particular theory, it may be hypothesized that during UV irradiation, hydrogen donors trap the highly reactive hydroxyl radical and possibly the photoexited species as shown below: ##STR1##
At the same time, but with the opposite effect, the known anti-reversion agents may react with 0.sub.2 and transition metals to generate other free radicals as shown below:
The rate of light reversion with natural sunlight is 100-1,000 times greater than reversion in the dark (ambient or thermal reversion). Therefore, the "dark reactions" [3] and [4] can be ignored during light aging. However, in the absence of UV light, the participation of hydrogen donors in the "dark reactions" may accelerate ambient or thermal reversion by increasing the concentration of free radicals.
Although retardation of light reversion is usually more critical than retardation of thermal reversion, an acceleration in thermal reversion is likewise undesirable. A need thus persists for an efficient and low cost paper manufacturing process which reduces both light-induced and thermal brightness reversion of bleached pulps containing lignin.